Metal-free polymeric carbon nitrides (PCNs) are promising photocatalysts for solar hydrogen production, but their structurephotoactivity relationship remains elusive. Here, we characterize two PCNs by dynamic nuclear polarization-enhanced solid-state NMR spectroscopy, that circumvents the need for specific labeling with either 13C- or 15N-enriched precursors. This allows rapid 1-D and 2-D data acquisition, providing insights into the structural contrasts of the PCNs. Compared to PCN_B with lower performance, PCN_P, the porous and more active photocatalyst, is richer in terminal Nhydrogens not associated with inter-polymer chains, which are further proposed to act as efficient carrier traps and reaction sites.